This invention relates to a blade of a gas turbine having a novel heat-resistant ceramic coating layer, and also to a method of producing such a blade.
In a power-generating gas turbine, in order to enhance the power-generating efficiency, it is essential to achieve a high operating temperature and also to reduce the amount of a cooling medium, and it has been earnestly desired to enhance the high-temperature durability of turbine blades. In addition, in order to reduce the running cost of the gas turbine, it has been essential to prolong the service life of the turbine blades. Under the circumstances, heat-resistant materials, having high-temperature strength and excellent reliability, have been developed, but the heat resistance of the material itself is limited. Therefore, a thermal barrier coating (hereinafter often referred to as "TBC") has been proposed in order to reduce the substrate metal temperature of the blade used under high temperature conditions, and by using the TBC in combination with the cooling of the blades, the substrate metal temperature can be made 50 to 200.degree. C. lower than that achieved without the TBC.
However, the TBC, used under severe thermal load conditions, is susceptible to damage such as the separation of a ceramic layer, and this damage is conspicuous particularly in a high-temperature gas turbine designed to achieve the enhanced power-generating efficiency. Therefore, various TBCs, having further improved durability, have been proposed, and examples of such prior art techniques are as follows.
(1) U.S. Pat. No. 4,503,130
In these prior art techniques, there are disclosed a TBC of a columnar structure having a columnar-crystal ceramic layer having a thermal stress relaxation function, and a TBC of a porous structure having a porous ceramic layer, and damage due to the thermal expansion difference between the ceramic layer and heat-resistant alloy constituting the substrate, is prevented.
More specifically, the ceramic layer is composed of a ZrO.sub.2 ceramic material which has low thermal conductivity and high thermal expansion, and has excellent stability at high temperature. In this case, when ZrO.sub.2 is used alone, damage due to a dimensional change, caused by phase transformation, is encountered, and therefore stabilized ZrO.sub.2, which is prevented from phase transformation by the addition of a crystal structure-stabilizing agent (such as Y.sub.2 O.sub.3, CaO, MgO, CeO.sub.2, Sc.sub.2 O.sub.3), or partially-stabilized ZrO.sub.2 is used. With respect to the structure of the ceramic layer, the columnar structure or the porous structure is used. The TBC comprises the ceramic layer and a primary coat (underlayer) for this ceramic layer, and as the primary coat or layer, there is used MCrAlY alloy (M is one of Co, Ni, Fe or a combination thereof), having excellent high temperature oxidation resistance and corrosion resistance, or MCrAlY alloy whose surface portion is made rich in Al. In one example, the surface of the primary layer (i.e., underlayer) is beforehand oxidized to form a thin layer of Al.sub.2 O.sub.3.
In the case of using such TBC in a blade of a high-temperature gas turbine, the TBC is applied to blades having a cooling function, so as to achieve a thermal barrier effect. More specifically, the temperature of combustion gas is high on the blades of the high-temperature gas turbine, and therefore the cooling performance of the blades is high, and the quantity of heat (heat flux), penetrating the blade, becomes large. As a result, in the TBC provided on the surface of the blade, the thermal barrier effect is enhanced because of the large heat flux, and the effect of the TBC is enhanced. As the heat flux increases, the temperature of the ceramic layer, constituting the outermost layer of the TBC, rises. As a result, the oxygen ion conductivity of the ZrO.sub.2 ceramic, which is its inherent property, becomes higher, and the supply of oxygen to the surface of the primary layer (underlayer) through the ceramic layer increases, so that an oxidation reaction at the surface of the primary layer is promoted. Here, the primary layer comprises the MCrAlY alloy layer, and this alloy layer has the function of preventing damage due to the oxidation reaction. Particularly at high temperatures, Al, contained in the alloy, is oxidized into Al.sub.2 O.sub.3, and this Al.sub.2 O.sub.3 forms the barrier layer to prevent the internal oxidation of the alloy itself. Incidentally, ceramic members, having a TBC similar in structure to the above TBC, are disclosed, for example, in Japanese Patent Unexamined Publication Nos. 6-256926 and 8-27559.
(2) Japanese Patent Unexamined Publication No. 5-279832
In this prior art technique, a MCrAlY alloy layer is formed on a substrate, and a porous ceramic layer of a molten particle laminate structure is formed by spraying on this alloy layer, and pores in this ceramic layer are filled with Al.sub.2 O.sub.3, and by doing so, the internal destruction due to thermal stress relaxation is prevented.
(3) Japanese Patent Unexamined Publication No. 8-20878
In this prior art technique, a MCrAlY alloy layer is formed on a substrate, and a porous ceramic layer of a molten particle laminate structure formed by spraying, or a ceramic layer of a columnar structure formed by electron beams is formed on this alloy layer, and a thin coating of Al.sub.2 O.sub.3 is formed on the surface of the ceramic layer, and by doing so, the oxygen permeability in the ceramic layer is lowered, thereby enhancing the durability.
However, the above prior art techniques have the following problems.
In the TBC of the prior art technique (1) and the ceramic member having this TBC, the Al.sub.2 O.sub.3 barrier layer is formed by Al contained in the MCrAlY alloy layer, and therefore under conditions in which the oxidation is promoted at high temperatures, the thickness of the Al.sub.2 O.sub.3 layer is increased by Al of the Al.sub.2 O.sub.3 layer or by diffusion of oxygen. Therefore, for example, when this material is used in a high-efficiency power-generating gas turbine having high operation temperature, since there occur severe thermal load conditions in which the starting operation, the longtime holding of the steady state, and the stop are repeated, damage is caused in the vicinity of the Al.sub.2 O.sub.3 layer by thermal stresses of the heat cycle due to the increase in thickness of the Al.sub.2 O.sub.3 layer, and the separation of the ceramic layer is liable to occur in the vicinity of the boundary between the ceramic layer and the primary layer (Al.sub.2 O.sub.3 layer), which leads to a fear that the thermal barrier effect, which is the originally-intended purpose, is not satisfactorily achieved. Namely, in the prior art technique (1), no consideration is given to the prevention of the thickening of the Al.sub.2 O.sub.3 layer due to the increased oxygen-ion-conductivity of the ZrO.sub.2 ceramic layer at high temperatures so as to suppress the separation of the ceramic layer.
Incidentally, it has been reported that such damage, developing in the vicinity of the Al.sub.2 O.sub.3 layer formed at the interface of the primary layer and the ceramic layer, can also occur in a TBC in which a ceramic layer is of a columnar structure and which has a thermal stress relaxation function (Report of ASME meeting, 1991-GT-40). Further, it has been reported that in a porous ceramic layer formed by plasma a spraying, a thickened Al.sub.2 O.sub.3 layer at the interface determines the lifetime (durability) of a TBC ("Study of Estimated lifetime of Gas Turbine Barrier Coating" in Research Report vol. 36, No. 3, p475 by No. 123 Small Committee Research Report of Heat Resistant Metallic Materials; 1996).
In the prior art technique (2), as described above for the prior art technique (1), no consideration is given to the prevention of the thickening of the Al.sub.2 O.sub.3 layer due to the increased oxygen-ion-conductivity of the ZrO.sub.2 ceramic layer at high temperatures so as to suppress the separation of the ceramic layer. It may be thought that the structure, in which the pores in the ceramic layer are filled with Al.sub.2 O.sub.3 for the purpose of preventing the internal destruction due to thermal stress relaxation, somewhat suppresses the thickening of the Al.sub.2 O.sub.3 layer due to the increased oxygen-ion-conductivity. However, since only the pores are filled with Al.sub.2 O.sub.3, its effect is insufficient. This prior art publication does not disclose the thickening of the Al.sub.2 O.sub.3 layer which thickening is caused by the enhanced oxygen ion conductivity of the porous ceramic layer.
In the prior art technique (3), in both of the porous ceramic layer of a molten particle laminate structure and the ceramic layer of a columnar structure, consideration is given to the prevention of the increase in the oxygen-ion-conductivity of the ZrO.sub.2 ceramic layer at high temperatures. However, when this coating layer is used for a blade of a gas turbine, a lot of dust, including iron rust contained in combustion gas, flows and impinges on the coating layer (this is known as "erosion"), and therefore the Al.sub.2 O.sub.3 layer on the surface thereof is scraped off by this dust, so that the satisfactory effect can not be obtained for a long period of time.